The present invention relates to a photosensitive member for electrophotography, and more particularly to a photosensitive member having a photoconductive layer containing a new binder polymer.
The photoconductive layers of photosensitive members for electrophotography were conventionally made from inorganic photoconductive materials such as selenium, cadmium sulfide, zinc oxide, etc. However, selenium not only needs to be recovered as a toxic material but also has poor heat resistance because it is crystallized by heat. Cadmium sulfide also needs to be recovered as a toxic material and has poor humidity resistance. As for zinc oxide, it has poor humidity resistance and insufficient durability for repeated copying. The, efforts have been made to develop new photoconductive layers.
Recently, research has been advanced on organic photoconductive compounds simply referred to as "OPC" for use in photosensitive members for electrophotography, and their use has been growing from year to year. The organic photoconductive compounds are superior to their inorganic counterparts in that they are lighter, easier to form into films, easier to manufacture into photosensitive members and even capable of providing transparent photosensitive members depending on the types employed.
Despite these advantages, organic photosensitive members for electrophotography composed of organic photoconductive compounds (referred to simply as "OPC photosensitive members") are inferior to their inorganic counterparts with respect to sensitivity and durability. Therefore, they have been used primarily in low-speed machines to date.
To improve the sensitivity and durability of the OPC photosensitive members, research has been carried out to develop new charge carrier generating materials. photoconductive compounds such as charge carrier transport materials, sensitizers, etc. On the other hand, binder polymers have not been well investigated so far despite the fact that they largely affect the properties of the resulting photosensitive members. As a matter of fact, most of the OPC photosensitive members practically used at present employ commercially available general-purpose polymers. Selection among a lot of commercially available polymers is made from the viewpoint of photosensitive properties and ease of production. Accordingly, these binder polymers are not necessarily suitable for having the photoconductive compounds fully exhibit their own properties. For instance, in those photosensitive members having photoconductive particles dispersed therein, the binder polymers are required to provide good stability to particle dispersion, and polyvinyl butyral is commonly used as such a binder. However, this binder is poor in the separation and injection of charge carriers and suffers from such problems as sensitivity decrease and residual potential increase. On the other hand, polyesters, polycarbonates, polystyrene, etc. are efficient in the separation and injection of charge carriers, but they fail to disperse photoconductive particles stably, resulting in the agglomeration of most photoconductive particles. Further, the dispersion stabilization treatment of such polymers leads to a decrease in sensitivity and electric properties such as residual potential, etc.
Thus, binder polymers having not only satisfactory properties such as electric properties necessary for photosensitive members but also high adaptability for manufacturing photosensitive members have not been found yet.